A SIM is an integrated circuit that securely stores information relating to a subscriber-based communications system, e.g. cellular telephony. The SIM stores, amongst other data, the service-subscriber key (International Subscriber Identity Module or ISMI) that is used to identify a subscriber on mobile telephony services such as on mobile phones and computers.
Typically, the SIM is held on a removable card (SIM card) provided by the subscriber's network operator, although it can be provided as a fixed, in-situ component or in theory as a file or program stored on internal memory. The removable form of SIM card is preferred as it allows subscribers to keep the same information, including telephone number, when changing portable device. For this purpose, portable devices are required to provide a dedicated hardware connector for the SIM card.
The use of dual SIM handsets is becoming common. Dual SIM handsets allow different users to share a particular handset without the first user having to remove their SIM card. Also, a single user may wish to make selected calls or use data services using different numbers or mobile networks. A typical example is where a first SIM is used for local calls and a second SIM is used for international calls.
There is a small number of handsets that currently provide two SIM connectors; these require additional hardware and, consequently, increase the size of handset. Existing handsets require users to manually change SIM cards. Further, the use of traditional SIM cards and their associated interfaces are limited in terms of the transmission rate, storage capacity and processing capacity that can be offered. Mobile network operators are therefore limited in terms of how they may customise complex services within SIM cards.
Many mobile handsets, data tablets and computers include an expansion card slot, e.g. a secure digital (SD) card slot, which allows users to expand storage capacity. SD cards are available in standard, mini or micro formats. Files in SD memory cards can be directly operated by heterogeneous platforms. In terms of expanding the hardware functionality of a handset, a number of existing standards exist, including the Secure Digital Input Output (SDIO) standard, although this standard requires specific hardware support at the host device and cannot be directly operated by heterogeneous platforms.